isl_pw_multi_aff_substitute: improve error handling
isl_pw_multi_aff_substitute: improve error handling

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

VanirLLVM/toolchain_isl,nicolasvasilache/isl,Distrotech/isl,simbuerg/isl,UBERTC/isl,BenzoSM/isl,serge-sans-paille/isl,BobSaget-Mod/libisl,serge-sans-paille/isl,BenzoSM/isl,cfx-next/toolchain_isl-upstream,SaberMod/isl-current,cfx-next/toolchain_isl-upstream,tobig/isl,crossbuild/isl,UBERTC/isl,nicolasvasilache/isl,PollyLabs/isl,inducer/isl-mirror,epowers/isl,SaberMod/isl-current,Meinersbur/isl,Meinersbur/isl,abduld/isl,UBERTC/isl,epowers/isl,BenzoSM/isl,serge-sans-paille/isl,simbuerg/isl,simbuerg/isl,inducer/isl-mirror,evaautomation/isl,abduld/isl,BobSaget-Mod/libisl,VanirLLVM/toolchain_isl,BenzoSM/isl,UBERTC/isl,crossbuild/isl,inducer/isl-mirror,nicolasvasilache/isl,BenzoSM/isl,PollyLabs/isl,evaautomation/isl,cfx-next/toolchain_isl-upstream,KangDroidSMProject/ISL,evaautomation/isl,serge-sans-paille/isl,pierrotdelalune/isl,pierrotdelalune/isl,Distrotech/isl,tobig/isl,abduld/isl,Distrotech/isl,SaberMod/isl-current,BobSaget-Mod/libisl,simbuerg/isl,KangDroidSMProject/ISL,pierrotdelalune/isl,epowers/isl,BobSaget-Mod/libisl,VanirLLVM/toolchain_isl,serge-sans-paille/isl,PollyLabs/isl,KangDroidSMProject/ISL,PollyLabs/isl,Distrotech/isl,tobig/isl,VanirLLVM/toolchain_isl,PollyLabs/isl,inducer/isl-mirror,pierrotdelalune/isl,cfx-next/toolchain_isl-upstream,KangDroidSMProject/ISL,epowers/isl,jleben/isl,cfx-next/toolchain_isl-upstream,nicolasvasilache/isl,pierrotdelalune/isl,Meinersbur/isl,Meinersbur/isl,BobSaget-Mod/libisl,crossbuild/isl,jleben/isl,VanirLLVM/toolchain_isl,SaberMod/isl-current,inducer/isl-mirror,abduld/isl,tobig/isl,Distrotech/isl,jleben/isl,crossbuild/isl,simbuerg/isl,jleben/isl,evaautomation/isl,epowers/isl,nicolasvasilache/isl,jleben/isl,KangDroidSMProject/ISL